1. Field of the Invention
The present invention relates to an inverter circuit, and more particularly to an inverter circuit that can minimize the distortion of an AC supply voltage output converted from an input DC power supply and provide a stabilized AC supply voltage.
2. Description of the Prior Art
An inverter circuit is a kind of device that converts a DC power supply into an AC power supply. The inverter circuit is typically provided in an electronic appliance such as a notebook computer, a portable television receiver, etc., and provides an AC power supply to an AC load by converting a DC power supply from a battery or the like into the AC power supply. For instance, the inverter circuit may be used for providing the AC power supply to a cold cathode florescent tube which is equipped in a notebook computer having a large liquid crystal display (LCD) screen.
A conventional inverter circuit includes a DC-to-DC converter for converting an input DC power supply into another DC supply voltage suitable to be converted into a desired AC power supply, and a DC-to-AC converter, coupled to the DC-to-DC converter, for converting the DC supply voltage into the AC power supply voltage.
FIG. 1 is a schematic circuit diagram of a conventional inverter circuit. Referring to FIG. 1, the conventional inverter circuit is provided with a DC-to-DC converter 10 for converting an input DC supply voltage into another DC supply voltage of a predetermined level, a DC-to-AC converter 30 for converting the DC supply voltage outputted from the DC-to-DC converter 10 into an AC supply voltage to provide the converted AC supply voltage to load RL, and a feedback loop F/B for detecting the voltage outputted from the DC-to-AC converter and feeding the detected voltage back to the DC-to-DC converter.
The DC-to-DC converter 10 comprises a transistor Q1 for switching the DC supply voltage inputted through an input terminal Vin, a pulse width modulation integrated circuit (PWM IC) 11 which provides a PWM pulse signal for controlling the on/off time of the transistor Q1 thereto, a diode D1 for rectifying the output voltage of the transistor Q1, and a coil L1 for smoothing the output voltage of the diode D1.
The DC-to-AC converter 30 comprises a transformer T1 having primary windings for receiving the DC supply voltage outputted from the DC-to-DC converter 10 and a secondary winding for developing the AC supply voltage, transistors Q2 and Q3, coupled to the primary windings of the transformer T1, for alternately controlling the input timing of the DC supply voltage being inputted to the transformer T1, a resistor R1 for controlling a current flow to the transistors Q2 and Q3, a capacitor C1 for resonance, and a capacitor C2 for filtering the DC component of the current induced in the secondary winding of the transformer T1.
The feedback loop F/B is composed of a resistor Rs, coupled to the output of the DC-to-AC converter 30, for sensing the output current of the DC-to-AC converter 30, a diode D2 and a capacitor C3 for rectifying and smoothing the difference voltage between both ends of the resistor Rs and feeding the rectified and smoothed voltage back to the PWM IC 11 in the DC-to-DC converter 10.
The operation of the conventional inverter circuit as constructed above will now be explained.
Referring to FIG. 1, the DC supply voltage inputted through the input terminal Vin is applied to the emitter of the transistor Q1. The PWM pulse signal produced from the PWM IC 11 is applied to the base of the transistor Q1. Accordingly, the transistor Q1 is turned on in high level periods of the PWM IC 11. The output voltage of the transistor is rectified and smoothed by the diode D1 and the coil L1, and the rectified and smoothed DC voltage is supplied to the DC-to-AC converter 30.
The width of the PWM pulse signal, which is applied from the PWM IC 11 to the base of the transistor Q1 to control the duty cycle of the transistor Q1, may be adjusted manually by a user or automatically adjusted in accordance with the output voltage level of the DC-to-AC converter 30 detected by the feedback loop F/B.
The DC voltage converted by the DC-to-DC converter 10 is applied to the base of the transistor Q2 through the resistor R1 in the DC-to-AC converter 30 to turn on the transistor Q2. When a predetermined time elapses after the transistor Q2 is turned on, the transistor Q2 is turned off due to the current supply capability limitations of the transformer's T1 primary winding. If the transistor Q2 is turned off, the base of the transistor Q3 becomes high, resulting in the transistor Q3 being turned-on. The on/off operation of the transistors Q2 and Q3 is repeated alternately.
Specifically, a positive sine wave voltage is developed in the secondary winding of the transformer T1 during a period when the transistor Q2 is turned on and the transistor Q3 is turned off. In contrast, a negative sine wave voltage is developed in the secondary winding of the transformer T1 during a period when the transistor Q1 is turned on and the transistor Q2 is turned off. As a result, a complete sine wave is developed in the secondary winding of the transformer T1 by the alternate on/off operation of the transistors Q2 and Q3.
The sine wave voltage is then supplied to the AC load RL through the capacitor C2. The load RL may be a cold cathode florescent tube which serves as a back lighting lamp in a notebook computer.
As the AC supply voltage is supplied from the DC-to-AC converter 30 to the load RL, the output current of the DC-to-AC converter 30 is sensed by the sensing resistor Rs in the form of a voltage. The voltage sensed by the sensing resistor Rs is rectified and smoothed by the diode D2 and the capacitor C3, respectively, and is fedback to the PWM IC 11 in the DC-to-DC converter 10. The PWM IC 11 controls the duty cycle of the PWM pulse signal outputted therefrom in accordance with the feedback voltage from the feedback loop F/B to control the on/off time of the transistor Q1.
However, the conventional inverter circuit as described above has the drawback in that switching operations of the DC-to-DC converter 10 and the DC-to-AC converter 30 are not synchronized with each other since the switching operation of the DC-to-DC converter 10 is performed by the PWM pulse signal provided from the PWM IC 11, while that of the DC-to-AC converter 30 is performed by the frequency which is determined by the capacitance of the capacitor C1 and the inductance of the primary winding of the transformer T1. This causes the distortion of the AC output waveform to be supplied to the load, such as the back lighting lamp, and causes the AC output voltage to become unstable, resulting in that the lamp cannot be kept in a uniform luminescent degree and the life time of the lamp is shortened.